How do I make a pure white plane that receives shadows?
There is multiple way,but I see you dont want complex node trees so I will tell you the easiest one.

Add a plane and make the following material:



It basically say that the plane the actual material is diffuse but the camera see it as a white color.
Now we have a pure white color.

Render tour image with :


Shadow pass. 
AO pass.
Object index pass.


Now we come to the post production,we multiply your shadows and AO to the image but we have to make sure we are applying only on the plane.

So we do as follow:

While The mask is the object.